User cannot see emails

User cannot see emails

My boss has had an ongoing issue for several years now where he cannot view other users emails sent from CRM. 

He is set up as a Sales Manager and an Administrator. 

His settings appear to be correct: 

Zoho Mail: Add On Activated

Incoming server: imap.gmail.com Port 993
TLS ticked

Outgoing server: smtp.gmail.com Port 465
TLS ticked

However if I try to enter his email password in outgoing settings we get "SERVER DISCONNECT' pop up on the screen. 

I have spoken to many people at Zoho about this in the past, it has stayed unresolved, and we really need to get this sorted ASAP. 

Can you please help me?